1. Field of the Invention
The embodiments relate to a method for driving an image pickup apparatus, and, more specifically, to a method for driving an image pickup apparatus capable of performing focus detection in an image pickup area.
2. Description of the Related Art
A remarkable development is currently seen in the field of image pickup apparatuses. An image pickup apparatus is known in which focus detection adopting a pupil division method is performed using an image sensor obtained by forming a micro-lens in each pixel of the image pickup apparatus (Japanese Patent Laid-Open No. 2001-124984).
According to Japanese Patent Laid-Open No. 2001-124984, the image sensor is provided at an expected imaging area of an imaging lens. In addition, each pixel in the image sensor includes a photoelectric conversion element A and a photoelectric conversion element B, and each photoelectric conversion element is arranged in such a way as to be substantially conjugate to a pupil of the imaging lens with the micro-lenses of the image sensor formed on an imaging lens side.
Here, the photoelectric conversion element A receives a light beam that has passed through a portion of the pupil of the imaging lens. On the other hand, the photoelectric conversion element B receives a light beam that has passed through a portion of the pupil different from the portion through which the light beam received by the photoelectric conversion element A has passed. During the focus detection, signals are independently read from the photoelectric conversion elements A and B of a plurality of pixels, and two images are generated by the light beams that have passed through the different positions of the pupil of the imaging lens. In addition, image information can be obtained by adding the signals of the two photoelectric conversion elements A and B.
Since the signals of the photoelectric conversion elements A and the signals of the photoelectric conversion elements B are sequentially read independently in Japanese Patent Laid-Open No. 2001-124984, the time at which the signals of the photoelectric conversion elements A are received and the time at which the signals of the photoelectric conversion elements B are received is different from each other.
More specifically, when a signal in a certain row is to be read, first, reset signals of the photoelectric conversion elements A are output. Next, light signals of the photoelectric conversion elements A are output. Similarly, reset signals of the photoelectric conversion elements B are output, and then light signals of the photoelectric conversion elements B are output. By this operation, a time difference of tens to hundreds of microseconds is generated between the signals of the photoelectric conversion elements A and the signals of the photoelectric conversion elements B. Therefore, an error is generated between the signals of the photoelectric conversion elements A and the signals of the photoelectric conversion elements B, which makes it difficult to increase the accuracy of the focus detection.